Head there and talk with him. He wants you to plant something on the Dainty Sload. This is because Captain Volf decided to go against their previous arrangement. Erikur will tell you to head to the Red Wave and talk with Sabine Nytte to get some Contraband. With it in hand, you need to head onto the Dainty Sload and plant it in Captain Volf's footlocker.
Head to the nearby East Empire Company Warehouse. Go out toward the ship that you see on the dock in the Northwest. Head out onto the dock. Dive into the water (or head onto the and talk with Sabine Nytte who will direct to the footlocker under the dock). Talking with Sabine, you can buy the bottle from her (for 1500). You can also pickpocket the key from her. Look underneath the dock to find the footlocker. In the footlocker you will just find Balmora Blue.
Surface and head to the Northeast. Get back on shore and then head for the Dainty Sload. Make your way over to the door that leads below decks. Start sneaking here to avoid the people walking around. Make your way down to the bottom of the ship then head to the eastern stairs. Use those to go up a level and into the second part of the ship.
Be sure to keep sneaking and do so carefully here. The First Mate is very good at noticing people.
Return to Solitude and talk with Erikur. He has already had things start and the Captain is now in prison. He thanks you for your efforts and assures you that he will open whatever doors that the Thieves' Guild needs.